1. Field of the Invention
This invention relates to a wafer-fetching sensing device for wafer storage apparatus, and more particularly to an apparatus that upgrades the wafer positioning detection from a linear sensing level to a plane sensing level for increasing the production yield.
2. Description of the Prior Art
In semiconductor industry, transportation and storage of wafers in the IC fabrication process usually involve a wafer storage apparatus for storing a plural number of wafers and a robot for fetching the wafer. The wafers are arranged, at a predetermined order, into the wafer storage apparatus before an automation process can begin. In a fabrication site, the wafer storage apparatus usually locates beside a workstation or a group of workstations, and includes a wafer access opening for the robot to access and fetch the wafer thereinside.
FIG. 1 illustrates a conventional wafer storage apparatus 1 which includes a housing 100 providing an interior accommodation space 11 for orderly storing a plurality of wafers 3 therein; for example, to store wafers 3 upon a plurality of movable racks 13. By providing the movable racks 13, the wafers 3 then can be sequentially fetched out by the robot, through an access opening 50 of the housing 100, where the access opening 50 is arranged parallel to a normal storage state of wafer 3.
In order to make sure that each wafer 3 in the wafer storage apparatus 1 can be fetched out through the access opening 50 safely and right at an acceptable access level 10 formed parallel to the normal storage state of wafers 3, a first sensor unit 5 is installed at the access opening 50 for sensing whether or not the wafer 3 is posed by the normal storage state at the access level 10. Generally, the access opening 50 and the access level 10 is coplanar.
Referring now to FIG. 2, the first sensor unit 5 includes a first emitter 17 located at the access opening 50 for sending a signal to a first receiver 15 along a first measuring line 101 on the access level 10. The first measuring line 101 may coincide with the center axis of the access opening 50. When the first sensor unit 5 detects the wafer 3 is properly positioned and leveled, the robot may proceed to start the fetching operation to move out the wafer 3 through the access opening 50.
However, there are some occasions in which a wafer 3 may be improperly positioned and may deviate from its normal state upon the rack 13. Upon occurring such an occasion, a wafer-fetching problem that the first sensor unit 5 fails to detect the misplacement may arise. FIG. 3 illustrates one of such occasions. Due to any possible operation error (e.g. by putting the wafer 3a with two lateral sides anchored at different levels of racks 13, the wafer 3a is tilted about the first measuring line 101 (also symbolized by the position of the first receiver 15) in the center of the access level 10, and forms an angle with the access level 10, but still keeps the center line of the tilted wafer 3a coinciding with the first measuring line 101. Under such a situation shown in FIG. 3, the first sensor unit 5 can still decide that the wafer 3a is posed at the normal storage state, and then proceeds to issue a normal signal for the robot to fetch out the wafer 3a. However, both lateral sides of the wafer 3a have deviated from the access level 10 and thus may result in the wafer 3a hitting the storage wall of the housing 10 or dropping to the floor, while being fetched out. It may cause damage (either minor or major) to the wafer 3a or even sequentially shut down whole production operation.